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Transcript
WARM UP: 1. Calculate the acceleration of a bag of softball gear weighing 50 N if pulled with a force of 25 N. 2. Calculate the force of a cars tires on the road if the car has a mass of 2000 kg and is traveling at a constant velocity of 60 mph north • Quiz TOMORROW! Free Body Diagrams and Net Force Terms used with Forces Net Force: the sum of all forces acting on an object. F Expressed in N. https://www.youtube.com/watch?v=YyJSlcIbd-s Terms used with Forces Balanced Forces net force on an object is zero, the object is in equilibrium—the forces are balanced. The object is either: 1. Not moving 2. Moving at a constant velocity Terms used with Forces UNBalanced Forces Whenever the forces do not cancel each other out. The object is ACCELERATING: 1. changing direction 2. changing speed Free Body F 1. Show all forces acting F Diagrams tension Hanging objects F air normal supporting Falling – terminal velocity - gliding 2. Think only about the objects acceleration F applied F friction Always opposes motion – direction of movement on one object 3. Length of vector matters F grav On every diagram 4. Be careful with constant velocity problems A physics book rests upon a level table. F norm F grav A physics book is pushed across the desk with a F norm rightward acceleration. But does not move F applied F friction F grav A physics book is pushed across the desk with a F norm rightward acceleration. F applied F friction F grav A physics book is pushed across the desk and moves with a constantF norm velocity. F applied F friction F grav A ball is moving upward after being kicked F grav An egg is free falling from a nest. F grav A car hits the brakes and is coasting F norm F friction F grav An elevator is rising at a constant velocity. F tens F grav • Determining Net Force: Free-body diagrams for four situations are shown below. The net force is known for each situation. However, the magnitudes of a few of the individual forces are not known. Analyze each situation individually and determine the magnitude of the unknown forces. If a = 1.5 m/s2 determine the net force and the mass of the car. • Unfortunately for Vanessa, the wheels on her suitcase are not working. She pulls on the strap in an effort to budge it from rest and drag it to the curbside check-in desk. Use force values to determine the net force, the mass and the acceleration of the suitcase. The values of the individual forces are: F = F = 207 N grav norm Ftens = 182 N Ffrict = 166 N. Which one(s) of the following force diagrams depict an object moving to the right with a constant speed? List all that apply. Newton’s 1st law He is providing a pushing force. Constant velocity Newton’s 1st law There is an equal and opposite friction force. Applied force friction Constant velocity Remember when the forces are balanced (net force is zero) he travels at constant velocity. Applied force friction Constant velocity Newton’s 2nd law Now lets imagine what happens if he pedals faster. Applied force friction Newton’s 2nd law His velocity changes (goes faster). He accelerates! Remember from that acceleration is rate of change of velocity. In other words Applied force acceleration = (change in velocity)/time friction acceleration Newton’s 2nd law Now imagine what happens if he stops pedalling. friction Newton’s 2nd law He slows down (decellerates). This is a negative acceleration. But he is still moving forward. friction Newton’s 2nd law So when the net force is greater than zero, an object accelerates (changes velocity) Mrs. Rice’s Porsche applied force friction An example We can use the diagram to determine his acceleration? Applied force (100 N) Mass of cyclist and bike = 100 kg Friction (60 N) An example Net force = 100 – 60 = 40 N FNET = ma 40 = 100a a = 0.4 m/s2 Mass of cyclist and bike = 100 kg Pushing force (100 N) Friction (60 N)